Buhler motors are usually picked up surplus- the new price would make your eyes water, as these motors are industrial quality with equivalent price tags!
Outfits like Proops and Model motors direct used to be the main source for these type of motors, alongside ham radio exhibitions. Nowadays they crop up more on ebay, especially the U.S site.
As such there are hundreds of different types of motors, some better documented than others, so you take a bit of a chance, which is probably why many modellers stick with motors from the likes of Graupner, MFA etc.
Other excellent makes worth looking out for are Maxon, Portescap, Pittman and Faulhaber.